Elevators are controlled to follow a flight (travel) profile which minimizes travel time within certain jerk, acceleration, and velocity constraints. The constraints are selected to ensure a comfortable ride. In practice, elevator vertical motion includes oscillations about the nominal trajectory (profile) that reduce ride comfort. These oscillations are primarily due to various spring/mass oscillation modes of the compliant rope between the elevator motor and the car. These oscillation modes are very lightly damped and thus can be set in motion by small disturbances that occur in flight. These small disturbances include passenger motion, rail joints, mechanical wear, torque ripple produced by the drive and motor, air pressure changes due to passing floor sills, other cars, and structural members in the hoistway, etc.
Elevator motion control is the mechanism by which the elevator is made to follow the nominal travel trajectory. Elevator motion control is typically accomplished using an elevator motion controller. In the elevator motion controller, the nominal profile to be followed by the elevator is input in terms of a dictated velocity of the elevator car along the profile. The dictated velocity is used to form the nominal commanded speed for the elevator motor. Near the end of flight, the position of the elevator car is measured and used to determine a distance to go estimate which is used to determine a correction to this nominal velocity command to ensure that the elevator lands (arrives and stops) at its desired destination in a smooth and controlled manner within a desired landing accuracy.
The motion controller typically includes a machine room motor velocity controller, which provides feedback of motor or sheave velocity in order to implement the motion command. The feedback of motor velocity to motor torque provides co-located damping of the oscillatory modes so that they are more quickly attenuated. In general, there will be some error in following the nominal profile because the oscillations are not attenuated as much as desired. The error is most critical at the end of the flight, where the error is termed "leveling error". The tracking and leveling errors decrease with the bandwidth of the motion control feedback loop and increase with acceleration and deceleration levels. Currently, the bandwidth is limited by the propagation delay through the rope.
In tall buildings, trajectory-following errors are worse because the long hoist rope is more compliant and there is a considerable time delay for the transmission of a motor motion perturbation in the machine room to propagate down the rope to the car. The speed of this tension wave in a typical elevator rope is 2500 to 3500 meters/sec. Thus there is approximately a 0.1 sec delay for a perturbation in the machine room to propagate to the car if the car is 300 meters below the machine room. The presence of this time delay in the motion control feedback loop limits its bandwidth, which limits how quickly the controller reacts to errors in following the nominal flight trajectory and to disturbances. This limitation has two impacts: (1) the elevator vertical oscillations cannot be as well attenuated; and (2) the accuracy to which the car can be made to follow a decelerating trajectory decreases. The taller the elevator rise, the greater the impact of time delay. To maintain accuracy at landing (e.g., to minimize leveling errors), the deceleration rate of the car has to be reduced for tall buildings. This increases floor-to-floor flight time and is therefore undesirable. Therefore, a need exists for an improved elevator motion controller which improves the attenuation of oscillations, without increasing travel times, particularly in buildings with long elevator shafts.
To accurately land, the elevator motion control needs to include some degree of position error feedback. A common way to accomplish this is to make the dictated velocity a function of distance-to-go. Although position feedback is needed to land accurately, it reduces the damping of the oscillatory modes. It is known that a high position gain (i.e., the slope or gain of a dictated speed vs. distance-to-go function) can cause instabilities. It is also known that lowering position gain increases flight time. The degree of position error feedback that can be allowed increases with the damping of the oscillatory modes. It is further known in the art that car acceleration feedback to the velocity command (provided to a drive and brake subsystem) increases this damping in modest size buildings. In tall buildings, this is not effective because of the relatively large time delay in propagating motion from the main motor to the car. Therefore, there further exists a need for improved attenuation of oscillations for improved position error feedback control.
In U.S. Pat. No. 5,750,945 there is described an elevator motion control system which compares a dictated travel path signal, indicative of a desired elevator travel profile, with a measured travel path signal, indicating actual elevator motion, and provides a motion command signal to appropriate circuitry. The frequency of the motion command signal is split into high and low frequency components, and an active force actuator, located at the elevator car, is used to implement the high frequency/low stroke portion of the motion command signal, while the elevator motor is used to implement the low frequency/high stroke portion of the motion command signal.
The active force actuator is located together with a passive damping device between a hitch and an elevator car frame or between the frame and the car. The active force actuator, or actuators, may be electromagnetic voice coils, the extension and contraction of which are provided by control signals applied thereto, or they may be hydraulic actuators, rotary motors with lead screws, or other suitable devices. In each instance the actuator is actively controlled in both directions, i.e., extension and retraction, to improve the vertical motion control of the elevator along its travel path. Such active control enables the actual motion of the elevator to closely track the elevator vertical travel command signal, by compensating for delays occasioned by the length of the elevator rope. However, the energy source for the respective active actuator typically includes a motor, a pneumatic or hydraulic pump, or a large electrical coil located on the elevator car to drive the respective actuator in both directions. Such arrangements typically are relatively heavy, noisy, and/or costly and may have limited reliability, thus creating a limitation to their overall usefulness in this particular environment. Therefore, there exists a need for further improvement in the type and control of the actuator associated with the elevator car and hitch for damping elevator car vertical oscillations.